The present invention arose from an effort to economically produce computer keyboards in a manner compatible with production plastic molding procedures. It also was developed to permit conversion of keyswitch plugers, when used in combination with yieldable domes, from a "quiet tactile" operation (where the plunger engages switching elements under it through the dome) to a "click tactile" operation (where the plunger directly engages the switching elements).
Reciprocating keyswitch plungers require accurate fixed guide surfaces for ease of operation when the keyswitch is manually depressed. In the simplified form of keyswitch to which this disclosure is addressed, the guide surfaces are present in a bearing block fixed to the keyboard framework and overlying the switches or other electronic devices controlled by movement of the plunger. The guide surfaces are located along the interior of a through bore formed in the bearing block. The through bore has an axial length adequate to properly support the axially movable plunger. The cross sectional configuration of the through bore complement the cross sectional configuration of the plunger.
Since molded plastic resins tend to shrink onto interior metal molds used to form bores, it is necessary to provide draft or clearance in the design of the bore walls to assure mold removal without unplanned difficulty. However, a drafted bore presents non-uniform clearance along the length of the bore and reduces the quality of the sliding fit between the plunger and bearing block.
The present discovery provides the sliding plunger of a keyswitch operator with bearing surfaces that are parallel to the plunger axis, assuring that it is accurately guided when the associated keytop is manually depressed. The through bore containing the bearing surfaces is also provided with drafted surfaces that facilitate mold removal.
Computer keyboard technologies involving the use of yieldable domes provide the designer with ability to modify keyswitch force curves and tactile feedback to the user. When using a dome overlying a switch, such as a printed membrane switch, one can either engage the switch through the dome or through a piston protruding through the center of the dome. In the first instance, which is known as "quiet tactile" operation, the force curve will be dependent upon the design of the dome. It will constitute a relatively "soft" curve, where the force required to depress the keytop will initially increase and then abruptly decrease as the walls of the dome are inverted. In the second instance, which is known as "click tactile" operation, the initial force curve will be dependent upon the design of the dome, plunger, and piston, and upon the spring used to bias the piston against the dome.
In the "click tactile" operation the plunger mounts an axially movable piston which has an end protruding through the dome actuator. As the plunger is depressed, a spring between the plunger and the piston is compressed until it exerts sufficient force upon the piston and dome to invert the dome walls. Once the dome walls begin to invert the dome exerts less pressure to resist the force of the spring and piston. As a result, the spring accelerates the piston towards the underlying membrane switch assembly. When the protruding portion of the plunger subsequently bottoms out on the membrane switch assembly, it produces a "click" sound. Further depression of the plunger is then absorbed by the spring.
The present keyswitch assembly has been designed to be readily converted from "quiet tactile" to "click tactile" operation, depending upon the requirements of the keyboard user or consumer.